The transition region above sunspots

Geoscience Letters, Feb 2018

Over decades, sunspots and their fine structures have been studied in detail at the photospheric level with different ground-based telescopes, as the surface of the Sun primarily emits light in the visible wavelengths. For a very long period, the upper atmosphere above the sunspot regions, especially the transition region (TR) above sunspots where the plasma emits light in the far ultraviolet (FUV) and extreme ultraviolet (EUV), has been poorly understood. In the past decades after the development of space instrumentations, FUV and EUV observations have uncovered many secrets of the TR above sunspots. In this paper, we present a brief review of research results about the TR structures and dynamics obtained through imaging and spectroscopic observations of sunspots in the past ~ 20 years. Though these observations have gathered remarkable and detailed information and greatly improved our understanding of the TR above the sunspots, paradoxically, they leave us with many new questions which should be answered in the future.

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The transition region above sunspots

Tian et al. Geosci. Lett. The transition region above sunspots Hui Tian 0 Tanmoy Samanta 0 Jingwen Zhang 0 0 School of Earth and Space Sciences, Peking University , Beijing 100871 , China Over decades, sunspots and their fine structures have been studied in detail at the photospheric level with different ground-based telescopes, as the surface of the Sun primarily emits light in the visible wavelengths. For a very long period, the upper atmosphere above the sunspot regions, especially the transition region (TR) above sunspots where the plasma emits light in the far ultraviolet (FUV) and extreme ultraviolet (EUV), has been poorly understood. In the past decades after the development of space instrumentations, FUV and EUV observations have uncovered many secrets of the TR above sunspots. In this paper, we present a brief review of research results about the TR structures and dynamics obtained through imaging and spectroscopic observations of sunspots in the past ~ 20 years. Though these observations have gathered remarkable and detailed information and greatly improved our understanding of the TR above the sunspots, paradoxically, they leave us with many new questions which should be answered in the future. Sunspots; Solar transition region; Chromosphere Introduction Sunspots are the darkest regions with the strongest magnetic fields and lowest temperatures on the surface of the Sun. Though observed by humanity for thousands of years, detailed investigation of the long-term evolution of sunspots started only 400 years ago, when Galileo Galilei invented the astronomical telescope. The number of sunspots strongly varies on a timescale of about 11 years, which is known as the sunspot cycle or solar cycle. Correspondingly, the occurring frequency of large-scale solar activities such as solar flares and coronal mass ejections also changes with such an 11-year cycle. As a result, the Earth’s magnetosphere and ionosphere are disturbed by the same cycle. The total electromagnetic radiation of the Sun is also closely related to the 11-year sunspot cycle. For instance, the extreme ultraviolet (EUV) and far ultraviolet (FUV) emission of the Sun varies significantly over a solar cycle. Modern high-resolution observations by large-aperture telescopes have revealed detailed substructures and short-term dynamics of sunspots. A sunspot often consists of a dark core termed umbra and a less dark penumbra surrounding the umbra. Numerous umbral dots are often found inside the dark core, while the most prominent structures in the penumbra are the so-called penumbral filaments. Systematic outward flows, termed Evershed flows (Evershed 1909) , are usually present in the penumbra. Some sunspots also have light bridges, which are bright lane-like structures dividing the umbra into two or more parts. It is believed that umbral dots, penumbral filaments, and light bridges are formed as a result of vigorous convective motions. For a detailed description of these substructures and dynamics, we refer to the comprehensive review by Solanki (2003) . Some of these substructures and dynamics have been well reproduced through radiative magnetohydrodynamic (MHD) simulations (see a review by Rempel and Schlichenmaier 2011) . Most studies of sunspots are focused on the photospheric structures and dynamics. For a very long period, the upper atmosphere in sunspot regions, especially the transition region (TR) above the sunspots, has been poorly understood. This is mainly due to the fact that there were only very limited numbers of TR observations in the sunspot regions. By definition, the TR is the interface region between the chromosphere and corona, where the temperature increases from roughly 2 × 104 to 8 × 105 K (e.g., Tian 2017) . TR probing relies mainly on imaging and spectroscopic observations of the EUV and FUV emission, primarily in the spectral range of 400– 1600 Å. In the past ~ 20 years, observations from mainly three EUV/FUV spectrographs have greatly improved our understanding of the TR. The first two instruments are the Solar Ultraviolet Measurements of Emitted Radiation (SUMER, Wilhelm et al. 1995) and Coronal Diagnostic Spectrometer (CDS, Harrison et al. 1995) onboard the Solar and Heliospheric Observatory (SOHO) launched in late 1995. The SUMER instrument has a very broad wavelength coverage, from roughly 660 to 1610 Å. Hundreds of strong TR lines have been observed by SUMER. The spatial resolution of SUMER observations is about 2′′–3′′. Unfortunately, most SUMER observations were performed in the first few years after the launch of SOHO, when the solar activity was relatively low. As a result, there are limited numbers of sunspot observations by SUMER. For more than one solar cycle, spectra of some strong TR lines have been acquired in many CDS observations. However, the CDS instrument was mainly designed for coronal studies and the spatial resolution of the data is of much poorer quality than that of SUMER. (...truncated)


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Hui Tian, Tanmoy Samanta, Jingwen Zhang. The transition region above sunspots, Geoscience Letters, 2018, pp. 4, Volume 5, Issue 1, DOI: 10.1186/s40562-018-0103-1